This invention relates to apparatus for exciting and regulating rotating alternating current generators and, more particularly, to a static control system for a self-excited variable speed generator.
Alternating current (a-c) generators or alternators are electrical machines having relatively rotatable magnetic core members forming a magnetic circuit. A field winding is commonly provided on one of the core members and a main or armature winding which developed the alternating current produced by the alternator is formed on another core member. Direct current (d-c) is supplied to the field winding to establish magnetic poles within the alternator. Within its normal operating range the magnitude of alternating current produced by the alternator is directly related to the relative rotational velocity between the armature and field winding and the magnitude of current in the field windings. Once the alternator is in operation a portion of the alternating current from the main winding can be rectified and used to provide excitation to the field winding. Alternators employing such techniques are well known and are referred to as self-excited machines.
The present invention is of particular utility in an electric propulsion system for traction vehicles such as, for example, off-highway vehicles of the earthmoving or mining type. Such a vehicle may include an articulated frame and a four-wheel drive. Both front and rear axles may be driven by an electrical system comprising a pair of variable speed reversible d-c motors which are energized by an alternator coupled to a diesel engine or other suitable prime mover. By appropriate manipulation of a speed control pedal, an operator can control the electric drive system so as to determine the vehicle speed. The speed control pedal acts as a throttle control to vary the speed of the diesel engine driving the generator and thus varies the electrical power output of the a-c generator.
A typical a-c generator for a traction vehicle may provide 750 amperes a-c RMS current at 1600 volts RMS and may require 300 amperes of d-c current excitation for its field winding. In such an application it is possible to siphon off a portion of the alternating current developed by the main winding in order to provide a relatively low voltage excitation to the field windings. However, if the alternator is started under substantially short circuit load conditions, such as in electric traction motor applications, the short circuit condition will prevent the main winding voltage from attaining a magnitude sufficient to allow the alternator to operate in a self-excited mode. A more practical method for implementing self-excitation is to provide an auxiliary or tertiary winding on the armature which produces an alternating current suitable for exciting the field winding after appropriate rectification. Such a system is shown in Canadian Pat. No. 812,936 issued May 13, 1969. In this patent the alternator is stationary and adapted to operate with a substantially constant relative rotational velocity and the alternating current output is regulated by control of the direct current excitation of the field winding. During initial start-up of the alternator, the field winding is excited by a battery which provides sufficient excitation under no-load conditions to cause current to be generated in the auxiliary winding without appreciably discharging the battery.
In a system in which the generator is operated over a range of rotational velocities and the alternating current produced by the generator is a function of the relative rotational velocity between the core members, it will be appreciated that at low speeds the energy produced by the auxiliary winding may be insufficient to provide field current at the desired level. Accordingly, at such low speeds the battery necessarily supplies at least part of the excitation for the field winding. If the generator is operated any appreciable percentage of time at low speeds, it will be apparent that the battery may become weakened and unable to supply adequate field excitation.
Although discharge of the battery may be avoided by drawing power at low armature velocities from a battery charger circuit such as a rotating exciter, this alternative does not provide a ready means of regulating the average magnitude of alternator field current. If the field current is allowed to stabilize at its own level, i.e., if the current is determined solely by the magnitude of available battery voltage, the power output of the alternator may exceed a desired level. Furthermore, if the battery circuit is continuously required to supply field excitation, the physical capability and size of the battery circuit becomes economically impractical, particularly in traction vehicle applications where equipment space is limited. Thus, it will be appreciated that the level of excitation supplied by the battery or other means should be regulated to prevent overexciting the alternator field winding and to blend the excitation energy withdrawn from the battery circuit with the excitation from the auxiliary winding in a manner to minimize the use of battery supplied excitation current.